Find a place to host your mp3's, browse to them on your bb, click on the mp3, click on the file and select "get link". When the song is playing via audio player navigate over to the menu, click on it and select "save". Check the "set as ringtone" box and then click on save. Your new ringtone should be set.
I have heard of free hosting sites, but i do not know of any. Maybe someone else on this forum can help you out with that part.
Use this site to upload your songs but it has to be cut to a smaller size so you can't do the entire song. Once you do that, point to the file and this site will give you the URL that you can send to your phone via email or manually browse to and then download to your phone.